Reference signal filter for interferometric system

ABSTRACT

The invention provides a method and apparatus for applying spatial filtering the optical beam of a free space optical coherence tomography (OCT) system substantially without problematic reflections back to the optical source. The invention teaches spatially filtering the reference beam of the OCT system which is typically designed to provide isolation of the optical source from undesirable optical feed-back, thereby achieving spatial filtering without generating undesirable reflections back to the optical source. Various embodiments are taught.

CROSS REFERENCES TO RELATED PATENTS OR APPLICATIONS

This utility application claims priority from U.S. provisional patentapplication No. 62/099,521; this application is also related to: U.S.provisional patent application No. 62/096,909 titled A polarized OCTsystem with improved SNR, and U.S. application Ser. No. 14/975,745, thecontents of which are incorporated herein by reference as if fully setforth herein; U.S. Pat. No. 7,526,329 titled Multiple ReferenceNon-invasive Analysis System; U.S. Pat. No. 7,751,862 titled FrequencyResolved Imaging System; and U.S. Pat. No. 8,310,681 titled Orthogonalreference analysis system with enhanced SNR.

FIELD OF THE INVENTION

The invention described and illustrated in this application relates tothe field of non-invasive imaging, analysis and measurement of targetssuch as tissue. Applications include, but are not limited to: imagingsub-dermal fingerprints in tissue; non-invasively analyzing tissue tomeasure the concentration of analytes such as the concentration ofglucose in tissue or tissue fluids; measuring the thickness ofparticular tissue layers, such as the thickness of the layer comprisedof the portion of the retina of an eye between the inner limitingmembrane (ILM) and the retinal pigment epithelium (RPE). In particularthe invention relates to improving the performance of the non-invasiveinterferometric technologies such as Optical Coherence Tomography (OCT)for imaging and analyzing tissue including, but not limited to, skintissue and retinal tissue.

BACKGROUND OF THE INVENTION

OCT is commonly used to image tissue for ophthalmic analysis, such asretinal imaging and analysis. OCT is also used to image skin tissue andOCT has been explored as a technique for measuring glucoseconcentration. For example U.S. Pat. No. 6,725,073 by Motamedi, et al.,titled “Methods for noninvasive analyte sensing” describes using OCT tomeasure glucose concentration.

An OCT system, such as the polarized multiple reference system OCTsystem, consistent with prior art, is depicted in FIG. 1. A broadbandoptical source, such as a super-luminescent diode (SLD) 101, is fibercoupled through a length of optical fiber 123 with a fiber collimator(not shown) and emits a collimated optical beam 102 which is transmittedthrough an optional polarizer 103 through a half-wave plate 104 andsplit by a polarized beam-splitter 105 into reference radiation 106 andprobe radiation 112. The length of optical fiber 123 acts as a spatialfilter to filter the optical radiation and thereby improve the qualityof interference signals.

The reference radiation 106 is transmitted through an attenuator 107 anda quarter wave plate 108 and then partially through a partial reflectivemirror 109 to a reference mirror 110 mounted on a oscillatingtranslation device 111, such as a voice coil or piezo device. Thecombination of the partial mirror 109 and the reference mirror 110generates multiple reference signals as described in the patentsincorporated herein by reference.

As the reflected reference radiation is transmitted back through thequarter wave plate 108, its polarization vector is rotated such that itwill be re-directed by the polarized beam-splitter 105 towards thedetection system depicted in the dashed box of FIG. 1.

The probe radiation 112 is transmitted through a second quarter waveplate 113 and through an anti-reflection coated blank that compensatesfor effects of the optical elements in the reference path. The proberadiation 112 is scattered by components in the target 115. Some of theprobe radiation is scattered back through the quarter wave plate 113where the double pass through the quarter wave plate 113 rotates itspolarization vector by ninety degrees thereby enabling this scatteredprobe radiation to be transmitted through the polarized beam-splitter105 towards the detection system.

The combined scattered probe radiation and reflected reference radiationis transmitted through an optional second half wave plate 116 to asecond polarized beam splitter 117 that reflects one set of componentsof the reflected reference and scattered probe radiation to a detector118 and transmits the orthogonal set of components of the reflectedreference and scattered probe radiation to a detector 119 therebyachieving balanced detection.

In some embodiments the optional second half wave plate 116 is notpresent but the second polarized beam splitter 117 is rotated forty fivedegrees about the optical beam 120 so that again the polarized beamsplitter 117 reflects one set of components of the reflected referenceand scattered probe radiation to a detector 118 and transmits theorthogonal set of components of the reflected reference and scatteredprobe radiation to a detector 119.

Operation of the OCT system is controlled by means of a control module121. The detected signals are processed by a processing module 122 toyield imaging and analysis of the target. The OCT system typicallyincludes one or more lens to focus the reference and probe radiation. Inthe embodiment depicted in FIG. 1 a single lens 124 focuses both thereference and the probe radiation. Such systems also typically includeone or more detector lenses.

In the embodiment depicted, the broadband optical source, such as asuper-luminescent diode (SLD) and lens combination 101, that emits thecollimated optical beam 102 includes a fiber that couples the SLD, via afiber collimator that delivers the collimated beam to the rest of theoptical system. In such fiber coupled systems the fiber acts as aspatial filter that delivers a high quality collimated beam. In fiberbased OCT systems, (in contrast to free space OCT systems) the beamsplitter function is also typically accomplished in fiber and there istherefore extensive fiber based spatial filtering.

However, in free space based OCT systems without fiber coupling betweenthe SLD source and the collimated beam, the lack of spatial filteringdegrades the performance of the OCT system. Approaches to accomplishspatial filtering by focusing the output of the SLD through a pin-hole,in addition to adding complexity, have the undesirable consequence ofreflecting light back to the SLD which can cause unacceptable noiserelated problems.

An optical isolator, typically using polarization components, can beinstalled between the SLD source and the pin-hole to substantiallyreduce undesirable light reflected back from the pin-hole to the SLD.However, this approach requires additional optical components withassociated cost and complexity.

While fiber based OCT systems have the advantage of fiber spatialfiltering of the optical beam, free space OCT systems have thesignificant advantage of being potentially very low cost and therebyoffering significant commercial advantage. There is therefore an unmetneed for a free space OCT system that has spatial filtering without theundesirable reflections back to the SLD and without the need foradditional optical components.

BRIEF SUMMARY OF THE INVENTION

The invention described herein provides a method and apparatus forapplying spatial filtering to the optical beam of a free space opticalcoherence tomography (OCT) system substantially without problematicreflections back to the optical source. The invention provides spatiallyfiltering the reference beam of an OCT system. The spatially filteredreference beam only forms interference signals with the portion of proberadiation corresponding to the spatially filtered reference beam,thereby effectively spatially filtering complete interference signals.Undesirable reflections due to spatially filtering the reference beamalso benefit from OCT designs that provide isolation of the opticalsource from undesirable optical feedback thereby achieving spatialfiltering without generating undesirable reflections back to the opticalsource.

The invention provides an improved free space optical coherencetomography system. In a system including a radiation source, at leastone probe beam and one reference beam, the inventive system providesthat the reference beam is spatially filtered, so that a portion of thespatially filtered reference beam forms at least one interference signalwith a portion of the probe radiation that corresponds to a portion ofthe spatially filtered reference beam, thereby effectively spatiallyfiltering the resulting interference signal.

The reference beam is spatially filtered by focusing the reference beamonto a surface that is reflective at a predetermined region and wherethat region has a diameter that is substantially less than the diameterof the collimated beam and is typically within a range of approximatelyequal to one to four times the dimension of the waist of the focusedreference beam.

In the preferred embodiments, the region is partially reflective, withreflectivity in the range of 80-95%.

In alternate embodiments, the partially reflective surface is a surfaceof a gradient index lens.

In other embodiments, the surface having a predetermined region ofreflectivity is that of the reference mirror. The region has a diameterwithin a range of approximately equal to one to four times the dimensionof the waist of the focused reference beam. In some of theseembodiments, such as Fourier domain OCT or conventional domain OCT,there is no partial mirror.

In alternate embodiments, in addition to the reference mirror region ofreflectivity, an additional or second surface also having apredetermined region of reflectivity in the range of 80-95% and having adiameter within a range of approximately equal to one to four times thedimension of the waist of the focused reference beam, is oriented toreceive reflections from the reference mirror surface.

In one embodiment, the invention provides an improved free space opticalcoherence tomography system, where the system includes, among othernecessary components, a radiation source producing reference and proberadiation, a detector, a pathway for reference radiation where saidpathway includes a partial minor (reflecting in the range of 80-95percent), a reference minor, a pathway for probe radiation, a means forcapturing and processing interferometric signals formed by reference andprobe radiation and received at the detector, and the improvementcomprises the partial mirror having a preselected region of itsreflecting surface so that radiation reflected between the preselectedregion of the partial mirror and the reference mirror is selected, andsubstantially all other radiation is filtered out of the referencesignal.

The preselected region of the partial mirror has a diameter within arange of approximately equal to one to four times a value of the waistdimension of the focused reference beam.

Alternatively, in an embodiment providing an improved free space opticalcoherence tomography system, where the system includes all functionalcomponents including a radiation source producing reference and proberadiation, a detector, a pathway for reference radiation, a referenceminor, a pathway for probe radiation, a means for capturing andprocessing interferometric signals formed by reference and proberadiation and received at the detector, the improvement comprises thereference minor having a preselected region of its reflecting surface sothat radiation reflected from the preselected region is selected, andsubstantially all other radiation is filtered out of the referencesignal. The preselected region has a diameter within a range ofapproximately equal to one to four times a value of the waist dimensionof a focused reference beam.

Various other embodiments are included in the invention, either depictedin the drawings or discussed in the detailed description

BRIEF DESCRIPTION OF THE DRAWINGS

Drawings intended as an aid to understanding the invention are:

FIG. 1 is an illustration of prior art depicting an OCT system using alength of fiber to spatially filter the optical beam.

FIG. 2 is an illustration of an embodiment of the invention depicting afree space OCT system that uses a single lens to focus both thereference beam and the probe beam.

FIG. 3 is an illustration of both a typical approach to collimation andconditioning of the optical beam emitted by the optical source.

FIG. 4, 4A through 4D inclusive, is an illustration of the modifiedpartial mirror optic depicting a reduced area reflective surface andoptional angled aspects of the partial mirror optic.

FIG. 4A shows the reference path as depicted in FIG. 2 and the opticelement that contains the partial minor in a free space OCT embodimentof the system.

FIG. 4B depicts in detail the optic element that contains the partialminor, in a side view.

FIG. 4C depicts in detail the optic element that contains the partialminor, in a frontal view.

FIG. 4D depicts—in a side view—an alternate embodiment providing andangled version of the optic element where the partial mirror is a flatsurface and the side of the optic is angled.

FIGS. 5, 5A and 5B inclusive, depicts an alternate embodiment accordingto the invention.

FIG. 5A depicts a sub-set of the system of FIG. 2 with a lens in thereference path and a lens in the probe path.

FIG. 5B depicts a subset of the system of FIG. 2 with a gradient indexlens in the reference path.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

The invention described herein provides a method, apparatus and systemfor spatially filtering the reference beam of an interferometric opticalsystem such as an optical coherence tomography (OCT) system and therebyimproving the signal to noise ratio of the associated interferencesignals. Since the spatially filtered reference beam forms aninterference signal only with the portion of probe radiation thatcorresponds to a spatially filtered portion of the reference beam,spatially filtering the reference beam thereby effectively spatiallyfilters the interference signal.

FIG. 1 has been discussed previously herein.

A preferred embodiment is depicted in FIG. 2 and, is in many respectsthe same as the system depicted in FIG. 1 (as indicated by the samenumbering). However the partial mirror 209 and optionally the referencemirror 210 are different and are described in more detail in FIG. 4.Also the optical source 201 that outputs a collimated optical beam 202is a free space configuration and is depicted in more detail in FIG. 3.

The dashed box 303 of FIG. 3 depicts an SLD 304 in a TO can (atransistor outline can)—(or, alternatively, a chip level device) whoseoptical output is collimated by a lens 305 which is typically anaspheric lens (or, alternatively, a lens system). In alternateembodiments, the collimated beam is made round by an anamorphic pair ofprisms 306 and 307 to output a collimated and round beam 308 (whichcorresponds to 202 of FIG. 2).

In FIG. 4A of sheet 4 the reference path of FIG. 2 is again shown andthe optic element that contains the partial mirror 209 (of FIG. 2) isdepicted in more detail in FIGS. 4B, 4C and (optionally) 4D. The opticelement 412 has a partial mirror surface or coating 413 on one side. Inthe preferred embodiment, the partially reflective mirror should have areflectivity in the range of 80% to 95%. The other surface 414 may beanti-reflection coated, uncoated or coated for a specific reflectivity(depending on the embodiment).

FIG. 4C depicts a key aspect of the present invention. FIG. 4C depictsan end on view of the partial mirror surface 413 of FIG. 4B and showsthat the partial mirror is confined to a small region 415, typically atthe center and typically (though not necessarily) round. The optimalmagnitude of this predetermined partially reflecting region 415 isdependent on the beam waist of the optical beam focused by the lens 124.

The diameter of the waist of the focused beam depends on the diameter ofthe collimated beam and the focal length of the focusing lens. Intypical applications the diameter of the waist of the focused beam is 20to 50 microns. Accordingly, the diameter of the predetermined reflectiveregion 415 is typically 20 to 60 microns. FIG. 4D shows the same view ofthe optic as FIG. 4B and shows the optional angled profile 417 of thesurface that does not contain the predetermined partially reflectingregion 416 (similar to region 415 of FIG. 4C). The angled profiledirects radiation that is not on the path to the reflective region 415out of the optical system and thereby prevents such radiation fromgenerating noise.

In alternate embodiments the lens 124 is a holographic lens.

In some embodiments the single lens 124 that focuses both the referenceand the probe radiation is replaced by two lenses. FIG. 5A depicts asub-set of the system of FIG. 2 where the single lens 124 of FIG. 2 isreplaced by lens 524 in the reference path and by lens 525 in the probepath. In some embodiments one lens is a gradient index lens; inalternate embodiments, both of the lenses are GRIN (gradient index)lenses.

The effect of having a reduced sized partial mirror is that iteffectively behaves as a pinhole and thereby spatially filters thereference beam. In the preferred embodiment, the radiation that is notpart of the spatially filtered reference radiation is substantiallyprevented from being propagated back to the optical source by thetechniques described in U.S. provisional patent application No.62/096,909 (that is incorporated herein by reference).

FIG. 4D depicts an alternate embodiment providing an angled version ofthe optic 412 where the partial mirror 416 is a flat surface, while therest of that side of the optic is angled, as indicated by 417. Theangled optic directs unwanted radiation out of the optical system so itcannot generate noise.

In some embodiments, depicted in FIG. 5B of Sheet 5, where the singlelens 124 (of FIG. 2) is replaced by a reference path lens 524 and aprobe path lens 525, the reference path lens 525 and the partial mirror525 are replaced by a GRIN (gradient index) lens 526 with the reducedarea partially reflective element 530 on a face 528 of the GRIN lenswhere the size and location of the reduced area partially reflectiveelement are such that only desired reference radiation is reflected bythe reduced area partially reflective element, thereby spatiallyfiltering the reference radiation. An advantage of this embodiment isthat the reduced area partially reflective element is physically fixedat a location in the waist of the focused beam and thereby lesssensitive to component alignment.

In other embodiments the optic with the reference mirror 210 containsthe reduced area reflective region and in some embodiments the rest ofthe reference mirror optic is also angled, similar to that depicted inFIG. 4D. In some embodiments, this modification to the reference mirror210 is an alternative to the similar modification to the partial mirror209, 413; in further embodiments, both the partial mirror and thereference minor have preselected regions of reflectivity where thereflectivity is of 80-95% in the case of the partial minor and areflectivity of approximately 100% in the case of the reference minor.Typically the reduced area reflective element (either partial or fullminor) is round in shape, however, other shapes, such as oval arepreferred in some embodiments.

While the preferred embodiment has been described with respect to apolarized version of a multiple reference OCT system, the invention canbe applied to any free space OCT system (polarized or non-polarized), orindeed to any free-space interferometric system. In the cases of Fourierdomain OCT, conventional time domain OCT or full field OCT, none ofwhich require a partial minor, the reference minor contains the reducedarea reflective element.

The invention is generally applicable to free space OCT system, andprovides a spatially filtered reference beam and wherein a portion ofthe spatially filtered reference beam forms at least one interferencesignal with only that portion of probe radiation that corresponds to theportion of the spatially filtered reference beam, thereby effectivelyspatially filtering the resulting interference signal.

In the preferred embodiment the reduced area reflective surface acts asan effective (or reflective) pinhole. In other embodiments one or moreactual pinholes are used to spatially filter the reference beam. Suchpinholes have a diameter of 20 to 60 microns. In some embodiments thereference beam is focused into a pinhole that only transmits a smallround portion of the waist of the focused reference beam therebyspatially filtering the reference beam.

Spatially filtering only the reference beam enables availing oftechniques to isolate the optical source from undesirable noisegenerating optical feedback that are described in U.S. provisionalpatent application No. 62/096,909 titled “A polarized OCT system withimproved SNR”, the contents of which are incorporated herein byreference.

In some embodiments the reference beam is spatially filtered by focusingthe reference beam onto a reflective surface that is reflective only ata small region related to the waist of the focused beam.

In embodiments where the surface having a predetermined reflectiveregion, and where the reflective region size is related to the waist ofthe focused beam, it 415 is a partially reflective surface, typicallybetween 80 and 95% reflective. In some embodiments the partiallyreflective surface 415 is a surface 413 of a GRIN lens; alternatively,the surface having a predetermined reflective region is a referencemirror surface 210.

Numerous modifications and variations could be made—by those skilled inthe art—within the spirit and scope of the invention; therefore to beunderstood that within the scope of the claims, the invention may bepracticed otherwise than as specifically described herein.

We claim: 1.-10. (canceled)
 11. An improved free space optical coherencetomography system, said system including a radiation source producingreference and probe radiation, a detector, a pathway for referenceradiation, a reference mirror, a pathway for probe radiation, aprocessor for processing interferometric signals formed by reference andprobe radiation and received at the detector, said improvementcomprising: at least one pinhole in the path of the reference radiation,wherein said pinhole has a diameter in the range of approximately 20 to60 microns, and wherein said pinhole spatially filters the referenceradiation.
 12. An improved free space optical coherence tomographysystem, said system including a radiation source producing reference andprobe radiation, at least one focusing lens, a detector, a pathway forreference radiation, a reference mirror, a pathway for probe radiation,a processor to process interferometric signals formed by reference andprobe radiation and received at the detector, said improvementcomprising: a pinhole in the path of said reference radiation such thatsaid pinhole transmits a portion of the waist of the focused referencebeam thereby spatially filtering the reference radiation
 13. The systemof claim 12, wherein substantially the entire area of said referencemirror surface is reflective.
 14. The system of claim 12, wherein saidpinhole is comprised of a coating on the surface of a partial mirror,said partial mirror surface facing said reflective surface of saidreference mirror.